CN111094885A

CN111094885A - Exhaust emission control system

Info

Publication number: CN111094885A
Application number: CN201880060101.4A
Authority: CN
Inventors: 亨里克·博曼
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2017-10-03
Filing date: 2018-10-03
Publication date: 2020-05-01
Also published as: GB2569841A; WO2019069074A2; GB201716132D0; KR20200062199A; US20200276537A1; GB2569841B; EP3692319A2; TW201927401A; BR112020005253A2; WO2019069074A3; EA202090537A1

Abstract

The invention discloses a method for producing formaldehyde. The process comprises feeding a feed stream comprising methanol to a reactor; converting the methanol to formaldehyde in the reactor using a mixed oxide catalyst to produce a process stream comprising formaldehyde; separating formaldehyde from the process stream to form a product stream comprising formaldehyde, and an off-gas stream; feeding at least a portion of the waste gas stream to a steam condenser to raise the temperature of the at least a portion of the waste gas stream to form a heated waste gas stream; and feeding the heated exhaust gas stream to a catalytic combustion bed for catalytically combusting components of the heated exhaust gas stream to form a combusted exhaust gas stream.

Description

Exhaust emission control system

Technical Field

The present invention relates to an emission control system for catalytically combusting components of a process exhaust stream. In particular, but not exclusively, the invention relates to an emission control system for use in a process for the production of formaldehyde, for example as formalin or UFC. The invention also relates to a method for producing formaldehyde, for example as formalin or UFC.

Background

Formaldehyde can be produced by the catalytic oxidative dehydrogenation of methanol. Processes for carrying out such production are known, for example from WO9632189 or US 2504402. The catalyst typically comprises a so-called "mixed oxide" catalyst comprising molybdenum and iron oxide. One well known process for producing formaldehyde is the Formox process supplied by Johnson Matthey. The Formox process involves the catalytic oxidative dehydrogenation of methanol over a mixed oxide catalyst. The Formox process is shown in FIG. 1. Methanol is mixed with air, vaporized and fed as a feed stream to the reactor where it is converted to formaldehyde. The process stream leaving the reactor is fed to an absorber, and formaldehyde is removed from the process stream and exits in the product stream at the bottom of the absorber, typically as formalin or UFC. A portion of the exhaust gas stream from the top of the absorber is fed to an emission control unit (the remainder being recycled, for example), where harmful components such as carbon monoxide, DME and methanol are catalytically combusted to produce a combusted exhaust gas stream that can be vented via a stack. In the present design, the combusted exhaust flow is used to preheat the exhaust flow entering the emission control system to the ignition temperature required for catalytic combustion. The present emission control system provides significant advantages over processes without such a system, but it is desirable to try to further improve the system in order to reduce capital costs and reduce pressure drop. This is particularly because emission control systems can be retrofitted to existing processes to increase their emission standards.

The present invention seeks to provide an improved emission control system and method for the production of formaldehyde.

Disclosure of Invention

According to a first aspect of the present invention, there is provided a process for the production of formaldehyde, the process comprising:

feeding a feed stream comprising methanol to a reactor;

converting methanol to formaldehyde in a reactor using a mixed oxide catalyst to produce a process stream comprising formaldehyde;

separating formaldehyde from the process stream to form a product stream comprising formaldehyde, and an off-gas stream;

feeding at least a portion of the waste gas stream to a steam condenser to increase the temperature of the at least a portion of the waste gas stream to form a heated waste gas stream; and

the heated exhaust gas stream is fed to a catalytic combustion bed to catalytically combust components of the heated exhaust gas stream to form a combusted exhaust gas stream.

By feeding the exhaust gas stream into the steam condenser to raise the temperature of the exhaust gas stream, the temperature of the heated exhaust gas stream as it is fed to the catalyst bed can be controlled so as to maintain a constant temperature of the heated exhaust gas stream entering the catalyst bed. This control is simpler than prior art systems where the combusted exhaust stream is used to heat the incoming exhaust stream because the steam can be controlled independently. For example, to maintain a certain temperature into the catalyst bed, a minimum required amount of steam with a minimum required steam pressure may be used. For example, a steam pressure of 14.6barg corresponds to a steam temperature of 200 ℃ and a steam pressure of 22.2barg corresponds to a steam temperature of 220 ℃. In an exemplary process, the catalyst bed comprises a catalyst including PPd and PPt catalysts available from Johnson Matthey Formox and operated at 12barg vapor pressure. This pressure advantageously corresponds to the minimum output steam pressure from a typical plant. This steam temperature will typically require a temperature differential in excess of the inlet temperature of the catalyst bed to allow for efficient heat exchange. Thus, in some embodiments, it is preferred that steam at a pressure of from 10 to 25barg, preferably from 15 to 25barg and most preferably from 17 to 20barg is fed to the steam condenser. Such vapor pressure may effectively heat the exhaust stream. The steam condensate may be collected and reused. Efficient heat transfer in the steam condenser may also allow for a lower pressure drop as the exhaust gas passes through the steam condenser. Reducing the pressure drop may be advantageous in providing a process that is cost and energy efficient, as the energy required to compress the gas entering the process, as well as the cost of that energy, may be significant. Preferably, the steam condenser and the catalyst bed are contained in a single vessel. Such an arrangement reduces the need for connecting piping and thus may further reduce the pressure drop of the system.

The product stream comprising formaldehyde is preferably formalin or UFC. The mixed oxide catalyst preferably comprises molybdenum and iron oxide. The conversion of methanol to formaldehyde and the separation of the product stream comprising formaldehyde in the reactor may be carried out, for example, according to the Formox process.

Preferably, the exhaust gas stream enters the bottom region of the steam condenser and flows upwardly through the steam condenser, and the heated exhaust gas stream flows upwardly through the catalyst bed. This may have several advantages. For example, catalyst beds in emission control systems are typically supported on catalyst screens. Flowing the heated exhaust gas stream upwardly through the catalyst bed means that the web is at the cooler inlet end of the catalyst bed. This is advantageous because the web does not have to withstand the same high temperatures and the risk of web failure is reduced. Therefore, a net having a desired mechanical strength can be provided more simply. An additional mesh may be required at the top of the catalyst bed to reduce catalyst movement due to the upward flow of heated exhaust gas, however the mesh is not subjected to the same forces as the mesh at the bottom of the catalyst bed and therefore need not be as strong. Therefore, it can be more easily designed to handle the high temperatures at the exit of the catalyst bed. In addition, flowing the exhaust stream in a single direction through the steam condenser and then onward through the catalyst bed can advantageously reduce the pressure drop. It may be particularly advantageous to have the exhaust gas stream flow upward, as the condensate (e.g., water) condensed in the steam condenser may then be allowed to flow downward in a counter-current manner under gravity, such that the hottest temperature is at the top of the steam condenser, where the heated exhaust gas stream is fed into the catalyst bed. Thus, preferably, the steam enters the steam condenser in a top region thereof and flows downwards through the steam condenser, condensing to form a condensate, and the condensate leaves the steam condenser in a bottom region thereof.

Preferably, the steam condenser is a shell-and-tube steam condenser, and the exhaust gas stream flows through the tube side of the steam condenser, while the steam is condensed in the shell side of the steam condenser. Such an arrangement may optimize pressure drop and heat transfer efficiency.

Preferably, the method further comprises:

the combusted off-gas stream is fed into a steam generator, wherein the combusted off-gas stream is cooled and steam is generated.

Thus, the method may use the heat from the combustion to generate steam, which may be used elsewhere in the process or elsewhere in the plant. For example, steam may be provided to a plant steam net. If steam of different pressure is used in the steam condenser and generated in the steam generator, providing steam to the plant steam network and using steam from elsewhere in the plant in the steam condenser may be an effective option, in a particularly preferred embodiment steam generated in the steam generator is used in the steam condenser. Therefore, the method preferably further comprises:

steam from the steam generator is fed into the steam condenser to raise the temperature of the exhaust gas stream in step d.

Heat is thus recovered from the combusted exhaust gas stream and used to heat the exhaust gas stream before it is fed to the catalyst bed, but the recovery and heating is done indirectly using steam. The steam is generated using heat from the combusted exhaust stream and is subsequently used to transfer that heat to the incoming exhaust stream. An advantage of such a system is that the pressure drop on the steam side of the process does not affect the total pressure drop of the formaldehyde production process. Thus, the heat transfer efficiency on the vapor side can be optimized without considering the effect of any pressure drop on the overall formaldehyde process. In any case the heat transfer in the condenser may be more efficient than in the gas-gas heat exchanger, which may be used if the exhaust gas stream is heated directly with the combusted exhaust gas stream. Furthermore, if more heat is needed, additional steam may be added, or if there is excess heat, some of the steam may be removed and used elsewhere, thus balancing heat integration more effectively. The system may also have advantages at start-up, as steam from another source may be used initially to heat the exhaust stream. This may eliminate the need for the cost of an electric heater to initiate heating of the emissions control system at startup.

Preferably, the steam generator is a shell and tube steam generator and the combusted exhaust gas stream flows through the tube side of the steam generator while steam is generated in the shell side of the steam generator. Advantageously, this may reduce the pressure drop of the combusted exhaust stream. In some embodiments, the steam generator may include a steam superheater, and in some embodiments, the steam generator may be a steam superheater.

Preferably, the steam condenser, catalyst bed and steam generator are housed within a single vessel. This may advantageously eliminate the pressure drop associated with the connection between the separation vessels. It may also provide a single unit that can be retrofitted to existing plants. It may also be mechanically advantageous to house the steam condenser, catalyst bed and steam generator within a single vessel, as this may eliminate the need for high temperature piping and flanges that would otherwise be required, particularly between the catalyst bed and the steam generator. The combusted exhaust gas stream exiting the catalyst bed in prior art systems may reach temperatures of about 550 ℃, and therefore any piping and flanges between the catalyst bed and the steam generator may need to handle such temperatures. If, for example, 22.2barg 220 ℃ steam is generated in the steam generator, the temperature of the combusted off-gas stream exiting the steam generator may be in the range of 230 ℃ to 245 ℃. Thus, if the catalyst bed and steam generator are housed within the same vessel, the piping and flanges of that vessel can be designed for temperatures of about 230 ℃ to 245 ℃, rather than 550 ℃, which can result in significant savings. Furthermore, when no piping and flanges are required between the catalyst bed and the steam generator in the present invention, it may be advantageous to increase the process temperature at the outlet of the catalyst bed to, for example, at least 580 ℃, preferably at least 590 ℃ and more preferably at least 600 ℃ at a reasonable cost. Such an increase may improve emission control of the process. Preferably, the steam generator produces steam having a pressure of from 10 to 25barg, more preferably from 15 to 25 barg. The steam generator may generate steam having a pressure of 17 to 20 barg.

Preferably, the steam condenser is a shell-and-tube steam condenser, wherein the exhaust gas stream flows through the tube side of the steam condenser, while the steam is condensed in the shell side of the steam condenser; and the steam generator is a shell and tube steam generator in which the combusted exhaust gas stream flows through the tube side of the steam generator and steam is generated in the shell side of the steam generator. Maintaining the exhaust gas stream and the combusted exhaust gas stream (i.e., the process exhaust gas stream) on the tube side of the steam condenser and steam generator can have significant advantages for scaling up the emissions control system. In such systems, the pressure drop may be maintained when scaling up the system by scaling up the number of pipes with the capacity demand. This is advantageously superior to prior art systems where the combusted exhaust gas stream is on the shell side and the exhaust gas stream is on the tube side, and scale up is more complicated.

Preferably, the combusted exhaust gas stream is fed through an expander section of a turbocharger to drive a compressor section of the turbocharger, prior to being fed to the steam generator, so as to pressurise an air stream fed into the process to form part of the feed stream. Using at least some of the energy from the combusted exhaust gas stream in a turbocharger for pressurizing the air stream fed into the process to form part of the feed stream, and thus the feed stream, is an efficient way to advantageously recover as much energy as possible from the combusted exhaust gas stream. Feeding the combusted exhaust gas stream to the turbocharger before feeding the combusted exhaust gas stream to the steam generator may be advantageous in optimally utilizing the high temperature combusted exhaust gas stream exiting the catalyst bed.

According to a second aspect of the present invention there is provided an emissions control system for catalytically combusting components of a process exhaust gas stream, the emissions control system comprising: a catalyst bed comprising a catalyst for catalytically combusting a component of a process exhaust stream; and a steam condenser having a tube side in fluid communication with the process exhaust gas flow inlet and the catalyst bed and a shell side in fluid communication with the steam inlet and the condensate outlet, such that in operation a process exhaust gas flow entering the process exhaust gas flow inlet is heated in the steam condenser prior to being fed into the catalyst bed.

Preferably, the emissions control system includes a vessel that houses both the catalyst bed and the steam condenser. Both the catalyst bed and the steam condenser in the same vessel can advantageously result in a less expensive device and a device with an advantageously low pressure drop across the emission control system.

Preferably, the process off-gas flow inlet is located in a bottom region of the vessel; the tube side of the steam condenser comprises a tube, preferably a vertical tube having an inlet end lower than an outlet end; the steam inlet is located in the top region of the shell side of the steam condenser; a condensate outlet is located in a bottom region of the steam condenser; and the catalyst bed is arranged above the steam condenser such that, in operation, a process exhaust gas stream entering the process exhaust gas flow inlet flows upwardly through the tube side of the steam condenser and upwardly through the catalyst bed, and steam entering the steam inlet flows downwardly through the shell side and condenses to form a condensate, wherein the condensate flows downwardly through the shell side and exits through the condensate outlet. Such a device can be operated and controlled particularly effectively, for example by controlling the level of condensate in the shell side.

Preferably, the emissions control system further comprises a steam generator having a tube side in fluid communication with the catalyst bed and the process exhaust gas flow outlet, and a shell side in fluid communication with the boiler feed water inlet and the steam outlet, such that, in operation, the process exhaust gas flow exiting the catalyst bed is cooled in the steam generator prior to exiting the process exhaust gas flow outlet, thereby converting boiler feed water entering through the boiler feed water inlet into steam exiting through the steam outlet. The steam outlet may be connected to a plant steam net to output steam to the plant. Preferably, the steam outlet is in fluid communication with the steam inlet of the steam condenser such that, in operation, steam generated in the steam generator is fed into the steam condenser to heat the process exhaust gas stream entering the process exhaust gas stream inlet. By providing a steam generator connected to the steam condenser, the apparatus can advantageously be used to transfer heat from the combusted process exhaust gas stream leaving the catalyst bed to the incoming process exhaust gas stream to be fed to the catalyst bed. An advantage of heat transfer via the steam generator and steam condenser is that the pressure drop on the process side of the emission control system can be kept low while still maintaining efficient heat transfer by the steam side design of the emission control system. Further, additional steam may be added or steam may be removed to balance the heat transfer required, as desired. To this end, the steam outlet of the steam generator may also be in fluid communication with a connector for connection to a steam network (e.g., a plant steam network).

Preferably, the emissions control system includes a vessel housing a steam condenser, a catalyst bed, and a steam generator. Combining all three stages in a single vessel advantageously reduces the cost of the apparatus and maintains a low pressure drop. Such a combination may also eliminate the need for high temperature (e.g., 600 ℃) flange connections between vessels. The temperature between the catalyst bed and the steam generator may be around 600 ℃, but if these components are in the same vessel, the only required connection is downstream of the steam generator where the temperature may be, for example, in the range of 230 ℃ to 245 ℃.

Preferably, the steam generator is located above the catalyst bed. In that way, the process exhaust gas streams flow one after the other up through all parts of the emission control system, thereby avoiding bends or other significant changes in direction that may increase the pressure drop.

Preferably, the emissions control system further comprises a turbocharger having a turbine-side inlet in fluid communication with the catalyst bed and a turbine-side outlet in fluid communication with the tube side of the steam generator such that, in operation, a process exhaust gas stream exiting the catalyst bed is fed into the tube side of the steam generator via the turbine side of the turbocharger. Thus, the energy in the process waste stream may be used to drive a turbine in a turbocharger to recover some of the energy in the process waste stream. The turbocharger may, for example, be configured to pressurize a stream (e.g., air stream) fed into the process, thereby reducing the new energy required to pressurize the feed stream.

Preferably, the emission control system is used in a method according to the first aspect of the invention. Advantageously, the emission control system is suitable for retrofitting into existing processes or plants for the production of formaldehyde. Assembling an emission control system according to the present invention can help a plant or process achieve better environmental performance without adversely affecting the pressure drop across the process.

Preferably, the emission control system is used to treat an exhaust gas stream in a process.

It will be appreciated that features described in relation to one aspect of the invention may be equally applicable to other aspects of the invention. For example, features described in relation to the process for producing formaldehyde of the present invention may be equally applicable to the emission control system of the present invention, and vice versa. It should also be understood that optional features may not apply to, and may be excluded from, certain aspects of the present invention.

Drawings

The invention will be further described, by way of example only, with reference to the following drawings, in which:

FIG. 1 is a diagram of a prior art Formox process for the production of formaldehyde;

FIG. 2 is a diagram of a process for producing formaldehyde according to an embodiment of the present invention;

FIG. 3 is an emissions control system according to an embodiment of the present invention;

FIG. 4 is an emissions control system according to another embodiment of the present invention;

FIG. 5 is an emissions control system according to another embodiment of the present invention; and is

FIG. 6 is an emissions control system according to another embodiment of the present invention.

Detailed Description

In the prior art Formox process 1 for the production of formaldehyde in FIG. 1, a fresh air stream 5 is passed through a pressurized blower 4 and then mixed with a recycle stream 22 to form a mixed stream 23, which is then fed into the vaporizer 10 via a recycle blower 3. In the vaporizer 10, the mixed stream 23 is mixed with the methanol stream 2 and vaporized using heat from the process stream 24 leaving the reactor 9. The resulting feed stream 25 is fed to a reactor 9, which in this embodiment is an isothermal reactor cooled by vaporization of a heat transfer fluid 32. The heat transfer fluid 32 is fed to the condenser 8 where it is condensed and steam 6 is generated from the boiler feed water 7, which is then returned to the reactor 9. In reactor 9, methanol in feed stream 25 reacts over an iron/molybdenum oxide catalyst to produce formaldehyde, which exits reactor 9 in process stream 24 comprising formaldehyde and the unreacted portion of feed stream 25. Process stream 24 passes through vaporizer 10 and is fed to absorber 11 where the heat in process stream 24 is used to vaporize feed stream 25. In the absorber 11, the process water 12 and optionally urea 13 flow downwards and the formaldehyde is stripped from the process stream 24 flowing upwards in the absorber 11. Water 12 and optionally urea 13 leave the bottom of the absorber together with formaldehyde as product stream 21. This product stream 21 is typically 55% formalin if only process water 12 is used, or UFC if urea 13 is used. The remainder of the process stream 24 exits the top of the absorber as an off-gas stream 26. The exhaust gas stream 26 is partially recirculated as a recirculation stream 22 and the remainder is sent to the emission control system 16. In the emission control system 16, the exhaust gas stream 26 is first heated in the preheater 14 using energy from the combusted exhaust gas stream 27 exiting the emission control system 16 and then combusted in the catalyst bed 15 with the catalyst comprising PPd and PPt to form the combusted exhaust gas stream 27. The combusted exhaust gas stream 27 leaving the catalyst bed 15 has a temperature of about 500 ℃ to 550 ℃ and is fed into the steam generator 20 and then back into the preheater 14 of the emission control system 16 to heat the incoming exhaust gas stream 26, where the combusted exhaust gas stream 27 is cooled and the boiler feed water 19 is converted to steam 18. The combusted exhaust gas stream 27 leaving the preheater 16 is sent to the stack 17.

The method according to the invention is shown in fig. 2. Fresh air stream 55 passes through a pressurization blower 54 and is then mixed with a recycle stream 72 to form a mixed stream 73, which is then fed into vaporizer 60 via recycle blower 53. In vaporizer 60, mixed stream 73 is mixed with methanol stream 52 and vaporized using heat from process stream 74 exiting reactor 59. The resulting feed stream 75 is fed to a reactor 59, which in this embodiment is an isothermal reactor cooled by vaporization of a heat transfer fluid 82. The heat transfer fluid 82 is fed into the condenser 58 where it is condensed and steam 56 is generated from the boiler feedwater 57, and then returned to the reactor 59. In reactor 59, methanol in feed stream 75 reacts over an iron/molybdenum oxide catalyst to produce formaldehyde, which exits reactor 59 in process stream 74 comprising formaldehyde and the unreacted portion of feed stream 75. Process stream 74 passes through vaporizer 60 and is fed to absorber 61 where the heat in process stream 74 is used to vaporize feed stream 75. In the absorber 61, the process water 62 and optionally urea 63 flow downwards and the formaldehyde is stripped from the process stream 74 flowing upwards in the absorber 61. Water 62 and optionally urea 63 exit the bottom of the absorber as product stream 71 along with formaldehyde. This product stream 71 is typically 55% formalin if only process water 62 is used, or UFC if urea 63 is used. The remainder of the process stream 74 exits the top of the absorber as an exhaust stream 76. The exhaust gas stream 76 is partially recirculated as a recirculation stream 72 and the remainder is sent to the emissions control system 66. In emission control system 66, exhaust stream 76 is first heated in a steam condenser 79. The exhaust stream 76 flows into the bottom of the steam condenser 79 and passes upward through the condenser 79. The steam 68 entering the steam condenser 79 condenses on the tubes and flows downward and out of the steam condenser 79 as condensate 80. The condensate 80 is collected and reused. The heated exhaust gas stream thus formed flows from the steam condenser 79 to the catalyst bed 65 having catalyst comprising PPd and PPt. In catalyst bed 65, components of the heated exhaust gas stream (such as carbon monoxide, DME, and methanol) are combusted to form a combusted exhaust gas stream, which enters steam generator 70. In the steam generator 70, the combusted exhaust gas stream is cooled and boiler feed water 69 is converted to steam 68. Steam 68 may be 12barg steam, which corresponds to the minimum export steam pressure from a standard plant. Steam 68 generated in steam generator 70 is fed to steam condenser 79 to raise the temperature of the incoming exhaust stream 76. Steam 68 may also be fed to or supplemented from a plant steam network 78. The combusted exhaust gas stream 77 exiting the steam generator 70 is sent to the stack 67. The temperature of stack 67 depends on the pressure of steam 68. For example, at a temperature difference of 25 ℃ (i.e., the temperature difference between the combusted off-gas stream 77 and steam), a stack 67 temperature of 225 ℃ corresponds to a steam 68 pressure of 14.6barg, and a stack 67 temperature of 245 ℃ corresponds to a steam 68 pressure of 22.2 barg. The steam condenser 79, catalyst bed 65 and steam generator 70 are all housed within a single vessel. The flanges and piping at the vessel outlet need to be adapted to handle the temperature of the stack 67, which is significantly lower than the 500 ℃ to 550 ℃ that the connection between the emission control system 16 and the steam generator 20 in the prior art process 1 of fig. 1 needs to handle. Advantageously, this may even allow higher process temperatures to be used at the outlet of the catalyst bed 65, such as 600 ℃, since, unlike the prior art, when the steam condenser 79, the catalyst bed 65 and the steam generator 70 are all housed within a single vessel, no piping and flanges are required at the outlet of the catalyst bed 65.

During startup, steam from elsewhere in the plant steam network 78 may be fed into the steam condenser 79, thereby eliminating the need for a separate electric heater for the emission control system 66.

In fig. 3, an emission control system 101 for catalytically combusting components of a process exhaust stream 105 is provided. The emission control system 101 includes a catalyst bed 111 that includes a catalyst for catalytically combusting components of the process exhaust stream 105. Catalysts typically include, for example, PPd and PPt supplied by Johnson Matthey Formox. The steam condenser 103 has a tube side in fluid communication with the process exhaust gas stream inlet and the catalyst bed 111 where the process exhaust gas stream 105 is fed into the emissions control system 101. The steam condenser 103 has a shell side in fluid communication with a steam inlet fed by a steam stream 112 and a condensate outlet 108. Downstream of the catalyst bed 111, the emissions control system 101 further includes a steam generator 102 having a tube side in fluid communication with the catalyst bed 111 and the process exhaust gas flow outlet 104, and a shell side in fluid communication with the boiler feedwater inlet 118 and the steam outlet 107. The vapor outlet 107 is in fluid communication with a vapor inlet stream 112 of the vapor condenser 103. The steam flow 106 is connected with a steam outlet 107 and a steam inlet flow 112 so that excess steam or make-up added steam can be removed as needed at any particular time.

The steam condenser 103, catalyst bed 111 and steam generator 102 are in a single vessel. The outlet temperature of the vessel is about 225 ℃ to 245 ℃, which is significantly lower than the temperature of 500 ℃ to 550 ℃ of the combusted exhaust gas stream exiting the catalyst bed 111. By feeding this stream directly from the catalyst bed 111 to the steam generator 102 in the same vessel, the need for high temperature piping and connections is eliminated. Removing the piping and connections in the high temperature region downstream of the catalyst bed 111 may allow for the use of higher process temperatures, such as 600 c, at that location in the process.

The steam condenser 103 is at the bottom of the vessel with the catalyst bed 111 above the steam condenser and the steam generator 102 above the catalyst bed. In operation, the process exhaust gas stream exiting the catalyst bed 111 is cooled in the steam generator 102 before exiting the process exhaust gas stream outlet 104, and steam generated in the steam generator 102 is fed into the steam condenser 103 to heat the process exhaust gas stream 105 entering the process exhaust gas stream inlet. Cold air 109 (which may be, for example, air at ambient temperature) or steam for heating 110 may also be fed to emission control system 101 to further control the temperature, if desired. The process exhaust gas stream 105 flows upward through the emissions control system 101, wherein a steam stream 112 is fed to the top of the steam condenser 103 and condensate is removed from a condensate outlet 108 at the bottom of the steam condenser 103. The steam condensed on the outside of the tubes of the steam condenser 103 will thus flow downwards under gravity towards the condensate outlet 108. The process exhaust gas stream 105 enters the bottom of the emission control system 101 and flows upward through the emission control system 101 in a relatively straight path, thereby avoiding unnecessary pressure drops. Compression costs can be significant in formaldehyde production, and even in the emission control system 101, any pressure drop must be accounted for in the initial compression of the feed gas. Therefore, avoiding unnecessary pressure drops may be important to produce a cost-effective process.

Thus, in operation, the incoming process exhaust gas stream 105 is heated by condensing steam in the steam condenser 103 before being combusted in the catalyst bed 111. The hot combusted exhaust gas stream leaving the catalyst bed 111 is cooled in the steam generator 102, thereby generating steam 107 which is in turn used to operate the steam condenser 103. The heat transfer efficiency on the steam side of the steam generator 102 and steam condenser 103 can be optimized without affecting the pressure drop on the process side, unlike prior art systems where heat is transferred directly between the exiting combusted exhaust stream and the entering process exhaust stream. When the steam generated in the steam generator 102 is insufficient to preheat the incoming process exhaust stream 105, for example during start-up, steam from another part of the plant may be fed to the steam condenser 103 via steam stream 106. This eliminates the need for a dedicated heater for activating the emission control system 101, thereby saving capital costs.

In fig. 4, an emission control system 201 is fed a process exhaust stream 205. At the upstream end of the emission control system 201, which is at the bottom of the vessel in which the emission control system is contained in fig. 4, there is a steam condenser 203. The tube side of the vapor condenser 203 is in fluid communication with the process exhaust stream 205 and the catalyst bed 211. The process exhaust stream 205 flows upward through the steam condenser 203 and through the catalyst bed 211 where the detrimental components of the stream are combusted to form a combusted exhaust stream. Downstream of the catalyst bed 211 there is a steam superheater 217. Downstream of the steam superheater 217 are a steam generator 202 and an economizer 223. The shell side of economizer 223 is fed boiler feed water 218 and has an outlet stream 216 connected to the shell side inlet of steam generator 202. The shell side of the steam generator 202 has an outlet steam flow 207 that is connected to a steam flow 206 through which steam can be removed or added as desired. After connection, the steam stream is split into a stream 214 that is fed into a steam superheater 217 to form superheated output steam 215 and a steam stream 212 that is fed into the steam condenser 203. The combusted exhaust gas stream exiting catalyst bed 211 passes through the shell side of steam superheater 217, through the tube side of steam generator 202, and then through the tube side of economizer 223, and then exits through combustion gas stream outlet 204, which is typically fed into a stack.

As with the embodiment of fig. 3, the process exhaust gas stream 205 is heated in the steam condenser 203 and then combusted in the catalyst bed 211 to combust the hazardous components and form a combusted exhaust gas stream. The combusted exhaust gas stream is then cooled in a steam superheater 217, steam generator 202 and economizer 223. The economizer 223 can be replaced with a low pressure steam generator. The economizer 223 or low pressure steam generator improves heat recovery efficiency by utilizing the low temperature heat remaining in the combusted exhaust gas stream after it passes through the steam generator 202. The boiler feedwater 218 fed to the shell side of the economizer 223 is heated by cooling of the combusted exhaust gas stream and fed to the shell side of the steam generator 202 where it is converted to steam. This steam is fed into a steam superheater 217 to form superheated steam 215 for output to other parts of the plant or to a steam condenser 203 to preheat the incoming process exhaust stream 205. Also, as with the embodiment in FIG. 3, the emissions control system 201 may be started using steam from elsewhere in the plant via steam flow 206, thereby eliminating the need for a dedicated start-up heater. Furthermore, the heat transfer efficiency on the vapor side of the emission control system 201 may be optimized without affecting the pressure drop on the process side.

Also, the emission control system 201 is housed in a single container. This may be advantageous as it reduces the need for inter-vessel connections, and in particular for high temperature inter-vessel connections. This may reduce capital costs, and may also reduce pressure drop, which may in turn reduce operating costs. Because the steam condenser 203 is at the bottom of the vessel and the process exhaust stream flows upward from the steam condenser 203 through the catalyst bed 211, the support screen on which the catalyst bed rests is at the cooler end of the catalyst bed 211. This may be advantageous because it may be easier to provide a support screen of sufficient strength when the support screen does not have to withstand the high temperatures at the outlet of the catalyst bed 211. A second mesh may be provided above catalyst bed 211 to prevent catalyst carryover in the combusted exhaust gas stream, but the mesh need not support the entire weight of catalyst bed 211.

In fig. 5, the emissions control system 301 includes a steam condenser 303, a catalyst bed 311, and a furnace steam superheater 319. Furnace steam superheater 319 can be used to generate superheated steam. Generating superheated steam in this manner may increase stack temperature because it is not possible to recover low temperature heat in furnace steam superheater 319. However, it has the advantage of generating superheated steam, which may be valuable elsewhere in the plant. The process exhaust gas stream 305 is preheated in the steam condenser 303 prior to being fed to the catalyst bed 311 where the harmful components are combusted to form a combusted exhaust gas stream. The combusted exhaust gas stream is fed into a furnace steam superheater 319 which generates superheated steam while cooling the combusted exhaust gas stream. The cooled combustion exhaust stream exits furnace steam superheater 319 via outlet 304 and is sent to the stack. Superheated steam produced in furnace steam superheater 319 can be fed to the shell side of steam condenser 303 to be used for preheating the incoming process exhaust gas stream 305. In this embodiment, furnace steam superheater 319 is located in a different vessel than the vessel containing steam condenser 303 and catalyst bed 311. Although there are advantages, for example in terms of reduced connections and hence reduced pressure drop, by placing all components in one vessel, there may be situations where it is preferable to use more than one vessel, for example due to space limitations when upgrading an existing process.

In the emission control system 401 of fig. 6, the catalyst bed 411 is located downstream of the steam condenser 403, and in this embodiment is located above the steam condenser. The process exhaust gas stream 405 flows upward through the tube side of the steam condenser 403 and then upward through the catalyst bed 411. As described above with respect to other embodiments, flowing the process exhaust gas stream 405 upward through the catalyst bed 411 provides advantages in terms of the temperature conditions to which the support screen for the catalyst bed 411 is exposed. Steam is fed to steam condenser 403 from steam inlet stream 412 near the top of the shell side, and condensate exits through condensate outlet 408 near the bottom of the shell side. Thus, the steam condenses on the tubes and flows downward under gravity to the condensate outlet 408. In doing so, steam heats the process exhaust stream 405 before it is fed to the catalyst bed 411.

The combusted exhaust gas stream exiting catalyst bed 411 is fed to turbocharger 420. In turbocharger 420, the pressure of the combusted exhaust gas stream is reduced and the feed stream to the process is pressurized. Typically, the combusted exhaust gas stream passes through an expander portion of the turbocharger 420 and the fresh air feed stream entering the process passes through a compressor portion of the turbocharger 420. In formaldehyde production processes, compression of the process gas can be a significant operating cost, and thus it can be advantageous to recover some of the energy in the combusted off-gas stream for compression of the feed stream.

The combusted exhaust gas stream passes from the turbocharger 420 through the tube side of the steam generator 402, on the shell side of which boiler feed water 421 is fed to produce steam 422. The steam thus produced is fed to a steam inlet stream 412, with additional steam being withdrawn or added as needed, and used to preheat the incoming process exhaust stream 405. Thus, the energy in the combusted exhaust gas stream is used to preheat the incoming process exhaust gas stream 405, but the heat is transferred indirectly using the steam generator 402 and steam condenser 403. As described above, this has several advantages, including a reduction in the pressure drop of the process exhaust stream and the opportunity to use replacement steam from another part of the plant during start-up, thereby eliminating the need for a dedicated start-up heater for emission control system 401. The inclusion of the turbocharger 420 allows the energy in the combusted exhaust gas stream to be efficiently utilized by using it in the turbocharger 420 when the combusted exhaust gas stream is at its hottest condition and then using it to generate steam in the steam generator 402 after it passes through the turbocharger 420.

The

emission control systems

101, 201, 301, 401 of fig. 3, 4, 5, and 6 may be used, for example, in the process 51 of fig. 2.

It will be appreciated that the embodiments set out above are examples of the invention and that those skilled in the art will appreciate that various modifications may be made within the scope of the invention. For example, the steam condenser and steam generator may be located in the same or different vessels, and the system may arrange the vessels horizontally or side by side. The process off-gas stream may flow downwardly or horizontally through some or all of the portions of the process.

Claims

1. A process for producing formaldehyde, the process comprising:

a. feeding a feed stream comprising methanol to a reactor;

b. converting the methanol to formaldehyde in the reactor using a mixed oxide catalyst to produce a process stream comprising formaldehyde;

c. separating formaldehyde from the process stream to form a product stream comprising formaldehyde, and an off-gas stream;

d. feeding at least a portion of the waste gas stream to a steam condenser to raise the temperature of the at least a portion of the waste gas stream to form a heated waste gas stream; and

e. feeding the heated exhaust gas stream to a catalytic combustion bed to catalytically combust components of the heated exhaust gas stream to form a combusted exhaust gas stream.

2. The process of claim 1, wherein the steam condenser and the catalyst bed are contained within a single vessel.

3. The method of any preceding claim, wherein the steam condenser is a shell and tube steam condenser and the exhaust gas stream flows through the tube side of the steam condenser while steam condenses in the shell side of the steam condenser.

4. The method according to any one of the preceding claims, wherein the method further comprises:

f. feeding the combusted off-gas stream into a steam generator, wherein the combusted off-gas stream is cooled and steam is generated.

5. The method of claim 4, wherein the steam generator is a shell and tube steam generator and the combusted exhaust gas stream flows through a tube side of the steam generator while steam is generated in a shell side of the steam generator.

6. A method according to claim 4 or 5, wherein the combusted exhaust gas stream is fed through an expander section of a turbocharger to drive a compressor section of the turbocharger, prior to being fed to the steam generator, so as to pressurise an air stream fed into the method to form part of the feed stream.

7. The method of any of claims 4 to 6, wherein the method further comprises:

g. feeding steam from the steam generator into the steam condenser to raise the temperature of the exhaust gas stream in step d.

8. The method of claim 7, wherein the steam condenser, the catalyst bed, and the steam generator are housed within a single vessel.

9. An emission control system for catalytically combusting components of a process exhaust stream, the emission control system comprising: a catalyst bed comprising a catalyst for the catalytic combustion of the component of the process exhaust stream; and a steam condenser having a tube side in fluid communication with a process exhaust gas stream inlet and the catalyst bed and a shell side in fluid communication with a steam inlet and a condensate outlet, such that in operation a process exhaust gas stream entering the process exhaust gas stream inlet is heated in the steam condenser prior to being fed into the catalyst bed.

10. The emissions control system of claim 9, wherein the emissions control system comprises a vessel housing both the catalyst bed and the vapor condenser.

11. The emissions control system of claim 9 or claim 10, wherein the emissions control system further comprises a steam generator having a tube side in fluid communication with the catalyst bed and a process exhaust gas stream outlet and a shell side in fluid communication with a boiler feed water inlet and a steam outlet, such that, in operation, the process exhaust gas stream exiting the catalyst bed is cooled in the steam generator prior to exiting the process exhaust gas stream outlet, thereby converting boiler feed water entering through the boiler feed water inlet into steam exiting through the steam outlet.

12. The emissions control system of claim 11, wherein the emissions control system comprises a vessel housing the steam condenser, the catalyst bed, and the steam generator.

13. The emissions control system of claim 11, wherein the emissions control system further comprises a turbocharger having an expander-side inlet in fluid communication with the catalyst bed and an expander-side outlet in fluid communication with the tube side of the steam generator, such that, in operation, the process exhaust gas stream exiting the catalyst bed is fed into the tube side of the steam generator via the expander side of the turbocharger.

14. The emissions control system of any one of claims 11 to 13, wherein the steam outlet is in fluid communication with the steam inlet of the steam condenser such that, in operation, steam generated in the steam generator is fed into the steam condenser to heat the process exhaust gas stream entering the process exhaust gas stream inlet.

15. An emissions control system according to any of claims 9 to 14, wherein the emissions control system is used in a method according to any of claims 1 to 8.

16. Use of an emission control system according to any of claims 9 to 14 for treating the exhaust gas stream in a method according to any of claims 1 to 8.